The objective of this thesis is to study two Basins of the Gulf of Guinea (GoG), namely the Dahomey and the Liberian Basins. These Basins are located in the northern part of the GoG, where oil and gas exploration has significantly increased in the last 10 years or so. We proposed geological descriptions of these two Basins. The key characteristics of the two models are the presence of channels and pinch-outs for depths of between 1 km and 2 km (these values are rescaled for our numerical purposes to 600- m and 700-m depths) and normal faults below 3 km (for our numerical purposes we use 1 km instead of 3 km). We showed that these models are consistent with the plate tectonics of the region, and the types of rocks and ages of rocks in these areas. Furthermore, we numerically generated seismic data for these two models and depth-migrated them. We then interpreted the migrated images under the assumption that the geologies are unknown. The conclusions of our interpretations are that we can see clearly the fault systems in both models. However, our results suggest that seismic interpretations of the channels and pinch-outs associated with the geology of the Dahomey and Liberian Basins will generally be difficult to identify. In these particular cases, we missed a number of channels and pinch-outs in our interpretations. The limited resolution of seismic images is the key reason for this misinterpretation.

Amplitude-versus-offset, AVO, approximations allow the estimation of various properties from pre-stack seismic gathers. Recently it has been suggested that fluid mobility is a controlling factor in pore pressure equalisation and can result in anomalous velocity dispersion in the seismic bandwidth. However, current approximations all assume an elastic subsurface and are unable to account for velocity dispersion. I have applied existing methodologies to a real dataset to qualitatively detect and interpret spectral amplitude anomalies. Three areas had AVO and spectral signature consistent with frequency-dependent AVO theory. The results suggest that it is feasible to measure such effects on real data in the presence of random noise. It would imply that the relaxation parameter, tau, is larger in the field than has been measured in water-saturated real and synthetic sandstones in the laboratory. I extended a two-term AVO approximation by accounting for velocity dispersion and showed how the resultant reflection coefficient becomes frequency-dependent. I then used this to measure P- and S-wave reflectivity dispersion using spectrally-balanced amplitudes. The inversion was able to quantify the affect of the P-wave velocity dispersion as an instantaneous effect on the reflection. NMO stretch was an issue at the far offsets and I limited myself to the near offsets and effectively measured only the P-wave reflectivity dispersion. I showed how the P-wave reflectivity dispersion signs depend on the AVO classification of the reflection whilst the magnitude depends on the crack density of my model. I showed how the effect of noise and thin-bed tuning can enter uncertainties into the interpretation of spectral anomalies. Whilst it is possible to detect frequency-dependent AVO signatures on pre-stack gathers, the interpretation remains non-unique. I have quantitatively measured a new physical property, reflectivity dispersion, from pre-stack seismic data. I have presented a method of detecting and measuring velocity dispersion in pre-stack gathers but there remain ambiguities in the interpretation of such results. The approach incorporates spectrally decomposed data in an extended AVO inversion scheme. Future work should investigate the application of the methodology to a real seismic dataset.

As últimas décadas viram ser publicados diversos trabalhos que se debruçaram sobre aspectos paleopatológicos de séries esqueléticas de sítios do tipo sambaqui. Tal como o litoral Sul e o restante do Sudeste, o seguimento paulista da costa brasileira apresenta grande quantidade de sítios do tipo sambaqui que guardam em si um grande potencial informativo. Entretanto, no levantamento realizado por Mendonça de Souza e colaboradores (2009) sobre os aspectos paleopatológicos dos sambaquis brasileiros, salienta-se a escassez de dados sobre as séries paulistas, que quando aparecem na literatura em geral restringem-se à duas ou três séries melhor estudadas. O presente estudo, de caráter exploratório, procurou gerar dados sobre o modo de vida dos grupos construtores de sambaquis do litoral de São Paulo, a partir dos padrões de ocorrência e distribuição dos sinais indicativos de osteoartrose. As articulações sinoviais de 48 indivíduos provenientes de dois sambaquis litorâneos da Baixada Santista foram analisadas quanto a frequência e distribuição dos seguintes sinais de osteoartrose: porosidade; eburnação; erosão; exostoses superficiais; labiamentos e osteófitos, considerando sua ocorrência diferenciada entre superfície e bordo da área articular. As séries estudadas apresentaram prevalências elevadas dos marcadores na maioria das áreas articulares destacadas. Os dados apontam para uma caracterização da osteoartrose nestas populações como um processo generalizado a nível populacional e individual e reforçam a interpretação vigente de que os grupos sambaquieiros devem ter exercido um domínio generalizado sobre as fontes alimentares e recursos necessários para a reprodução da vida material cotidiana. / The past decades witnessed the publication of several papers that dealt with paleopathological aspects of skeletal series of shellmoun-builters. As the southern coast and the rest of the Southeast Brazilian coast, the large number of shellmounds located in the São Paulo portion beheld information of great potential. However, in the study conducted by Mendonça de Souza et al. (2009) the scarcity of paleopathological data on the São Paulo series was highlighted, which when presented in the literature are generally restricted to the two or three best studied series. This exploratory study aimed to generate data about lifestyle of the shellmound-builters groups of the coast of São Paulo from the patterns of occurrence and distribution of signs indicative of osteoarthritis. The synovial joints of 48 individuals from two coastal sambaquis of Baixada Santista were analyzed for frequency and distribution of the following signs of osteoarthritis: pitting; eburnation; erosion; superficial new bone formation; lipping and osteophytes, considering its differentiated occurrence between surface and margin of the articular area. A high prevalence of injuries was noticed in most prominent articular areas. The data suggests a characterization of osteoarthrosis in these populations as a generalized process at the population and individual level.

As últimas décadas viram ser publicados diversos trabalhos que se debruçaram sobre aspectos paleopatológicos de séries esqueléticas de sítios do tipo sambaqui. Tal como o litoral Sul e o restante do Sudeste, o seguimento paulista da costa brasileira apresenta grande quantidade de sítios do tipo sambaqui que guardam em si um grande potencial informativo. Entretanto, no levantamento realizado por Mendonça de Souza e colaboradores (2009) sobre os aspectos paleopatológicos dos sambaquis brasileiros, salienta-se a escassez de dados sobre as séries paulistas, que quando aparecem na literatura em geral restringem-se à duas ou três séries melhor estudadas. O presente estudo, de caráter exploratório, procurou gerar dados sobre o modo de vida dos grupos construtores de sambaquis do litoral de São Paulo, a partir dos padrões de ocorrência e distribuição dos sinais indicativos de osteoartrose. As articulações sinoviais de 48 indivíduos provenientes de dois sambaquis litorâneos da Baixada Santista foram analisadas quanto a frequência e distribuição dos seguintes sinais de osteoartrose: porosidade; eburnação; erosão; exostoses superficiais; labiamentos e osteófitos, considerando sua ocorrência diferenciada entre superfície e bordo da área articular. As séries estudadas apresentaram prevalências elevadas dos marcadores na maioria das áreas articulares destacadas. Os dados apontam para uma caracterização da osteoartrose nestas populações como um processo generalizado a nível populacional e individual e reforçam a interpretação vigente de que os grupos sambaquieiros devem ter exercido um domínio generalizado sobre as fontes alimentares e recursos necessários para a reprodução da vida material cotidiana. / The past decades witnessed the publication of several papers that dealt with paleopathological aspects of skeletal series of shellmoun-builters. As the southern coast and the rest of the Southeast Brazilian coast, the large number of shellmounds located in the São Paulo portion beheld information of great potential. However, in the study conducted by Mendonça de Souza et al. (2009) the scarcity of paleopathological data on the São Paulo series was highlighted, which when presented in the literature are generally restricted to the two or three best studied series. This exploratory study aimed to generate data about lifestyle of the shellmound-builters groups of the coast of São Paulo from the patterns of occurrence and distribution of signs indicative of osteoarthritis. The synovial joints of 48 individuals from two coastal sambaquis of Baixada Santista were analyzed for frequency and distribution of the following signs of osteoarthritis: pitting; eburnation; erosion; superficial new bone formation; lipping and osteophytes, considering its differentiated occurrence between surface and margin of the articular area. A high prevalence of injuries was noticed in most prominent articular areas. The data suggests a characterization of osteoarthrosis in these populations as a generalized process at the population and individual level.

Les expériences sismiques actives sont largement utilisées pour caractériser la structure de la subsurface. Les méthodes dites d’analyse de vitesse par migration ont pour but la détermination d’un macro-modèle de vitesse, lisse, et contrôlant la cinématique de propagation des ondes. Le modèle est estimé par des critères de cohérence d’image ou de focalisation d’image. Les images de réflectivité obtenues par les techniques de migration classiques sont cependant contaminées par des artefacts, altérant la qualité de la remise à jour du macro-modèle. Des résultats récents proposent de coupler l’inversion asymptotique, qui donne des images beaucoup plus propres en pratique, avec l’analyse de vitesse pour la version offset en profondeur. Cette approche cependant demande des capacités de calcul et de mémoire importantes et ne peut actuellement être étendue en 3D.Dans ce travail, je propose de développer le couplage entre l’analyse de vitesse et la migration plus conventionnelle par point de tir. La nouvelle approche permet de prendre en compte des modèles de vitesse complexes, comme par exemple en présence d’anomalies de vitesses plus lentes ou de réflectivités discontinues. C’est une alternative avantageuse en termes d’implémentation et de coût numérique par rapport à la version profondeur. Je propose aussi d’étendre l’analyse de vitesse par inversion au domaine des données pour les cas par point de tir. J’établis un lien entre les méthodes formulées dans les domaines données et images. Les méthodologies sont développées et analysées sur des données synthétiques 2D. / Active seismic experiments are widely used to characterize the structure of the subsurface. Migration Velocity Analysis techniques aim at recovering the background velocity model controlling the kinematics of wave propagation. The first step consists of obtaining the reflectivity images by migrating observed data in a given macro velocity model. The estimated model is then updated, assessing the quality of the background velocity model through the image coherency or focusing criteria. Classical migration techniques, however, do not provide a sufficiently accurate reflectivity image, leading to incorrect velocity updates. Recent investigations propose to couple the asymptotic inversion, which can remove migration artifacts in practice, to velocity analysis in the subsurface-offset domain for better robustness. This approach requires large memory and cannot be currently extended to 3D. In this thesis, I propose to transpose the strategy to the more conventional common-shot migration based velocity analysis. I analyze how the approach can deal with complex models, in particular with the presence of low velocity anomaly zones or discontinuous reflectivities. Additionally, it requires less memory than its counterpart in the subsurface-offset domain. I also propose to extend Inversion Velocity Analysis to the data-domain, leading to a more linearized inverse problem than classic waveform inversion. I establish formal links between data-fitting principle and image coherency criteria by comparing the new approach to other reflection-based waveform inversion techniques. The methodologies are developed and analyzed on 2D synthetic data sets.

Le principale objectif de l'imagerie sismique pétrolière telle qu'elle est réalisée de nos jours est de fournir une image représentative des quelques premiers kilomètres du sous-sol. Cette image permettra la localisation des structures géologiques formant les réservoirs où sont piégées les ressources en hydrocarbures. Pour pouvoir caractériser ces réservoirs et permettre la production des hydrocarbures, le géophysicien utilise la migration-profondeur qui est un outil d'imagerie sismique qui sert à convertir des données-temps enregistrées lors des campagnes d'acquisition sismique en des images-profondeur qui seront exploitées par l'ingénieur-réservoir avec l'aide de l'interprète sismique et du géologue. Lors de la migration profondeur, les évènements sismiques (réflecteurs,…) sont replacés à leurs positions spatiales correctes. Une migration-profondeur pertinente requiert une évaluation précise modèle de vitesse. La précision du modèle de vitesse utilisé pour une migration est jugée au travers l'alignement horizontal des évènements présents sur les Common Image Gather (CIG). Les évènements non horizontaux (Residual Move Out) présents sur les CIG sont dus au ratio du modèle de vitesse de migration par la vitesse effective du milieu. L'analyse du Residual Move Out (RMO) a pour but d'évaluer ce ratio pour juger de la pertinence du modèle de vitesse et permettre sa mise à jour. Les CIG qui servent de données pour l'analyse du RMO sont solutions de problèmes inverses mal posés, et sont corrompues par du bruit. Une analyse de l'incertitude s'avère nécessaire pour améliorer l'évaluation des résultats obtenus. Le manque d'outils d'analyse de l'incertitude dans l'analyse du RMO en fait sa faiblesse. L'analyse et la quantification de l'incertitude pourrait aider à la prise de décisions qui auront des impacts socio-économiques importantes. Ce travail de thèse a pour but de contribuer à l'analyse et à la quantification de l'incertitude dans l'analyse des paramètres calculés pendant le traitement des données sismiques et particulièrement dans l'analyse du RMO. Pour atteindre ces objectifs plusieurs étapes ont été nécessaires. Elles sont entre autres :- L’appropriation des différents concepts géophysiques nécessaires à la compréhension du problème (organisation des données de sismique réflexion, outils mathématiques et méthodologiques utilisés);- Présentations des méthodes et outils pour l'analyse classique du RMO;- Interprétation statistique de l’analyse classique;- Proposition d’une approche stochastique;Cette approche stochastique consiste en un modèle statistique hiérarchique dont les paramètres sont :- la variance traduisant le niveau de bruit dans les données estimée par une méthode basée sur les ondelettes, - une fonction qui traduit la cohérence des amplitudes le long des évènements estimée par des méthodes de lissages de données,- le ratio qui est considéré comme une variable aléatoire et non comme un paramètre fixe inconnue comme c'est le cas dans l'approche classique de l'analyse du RMO. Il est estimé par des méthodes de simulations de Monte Carlo par Chaîne de Markov.L'approche proposée dans cette thèse permet d'obtenir autant de cartes de valeurs du paramètre qu'on le désire par le biais des quantiles. La méthodologie proposée est validée par l'application à des données synthétiques et à des données réelles. Une étude de sensibilité de l'estimation du paramètre a été réalisée. L'utilisation de l'incertitude de ce paramètre pour quantifier l'incertitude des positions spatiales des réflecteurs est présentée dans ce travail de thèse. / The main goal of the seismic imaging for oil exploration and production as it is done nowadays is to provide an image of the first kilometers of the subsurface to allow the localization and an accurate estimation of hydrocarbon resources. The reservoirs where these hydrocarbons are trapped are structures which have a more or less complex geology. To characterize these reservoirs and allow the production of hydrocarbons, the geophysicist uses the depth migration which is a seismic imaging tool which serves to convert time data recorded during seismic surveys into depth images which will be exploited by the reservoir engineer with the help of the seismic interpreter and the geologist. During the depth migration, seismic events (reflectors, diffractions, faults …) are moved to their correct locations in space. Relevant depth migration requires an accurate knowledge of vertical and horizontal seismic velocity variations (velocity model). Usually the so-called Common-Image-Gathers (CIGs) serve as a tool to verify correctness of the velocity model. Often the CIGs are computed in the surface offset (distance between shot point and receiver) domain and their flatness serve as criteria of the velocity model correctness. Residual moveout (RMO) of the events on CIGs due to the ratio of migration velocity model and effective velocity model indicates incorrectness of the velocity model and is used for the velocity model updating. The post-stacked images forming the CIGs which are used as data for the RMO analysis are the results of an inverse problem and are corrupt by noises. An uncertainty analysis is necessary to improve evaluation of the results. Dealing with the uncertainty is a major issue, which supposes to help in decisions that have important social and commercial implications. The goal of this thesis is to contribute to the uncertainty analysis and its quantification in the analysis of various parameters computed during the seismic processing and particularly in RMO analysis. To reach these goals several stages were necessary. We began by appropriating the various geophysical concepts necessary for the understanding of:- the organization of the seismic data ;- the various processing ;- the various mathematical and methodological tools which are used (chapters 2 and 3). In the chapter 4, we present different tools used for the conventional RMO analysis. In the fifth one, we give a statistical interpretation of the conventional RMO analysis and we propose a stochastic approach of this analysis. This approach consists in hierarchical statistical model where the parameters are: - the variance which express the noise level in the data ;- a functional parameter which express coherency of the amplitudes along events ; - the ratio which is assume to be a random variable and not an unknown fixed parameter as it is the case in conventional approach. The adjustment of data to the model done by using smoothing methods of data, combined with the using of the wavelets for the estimation of allow to compute the posterior distribution of given the data by the empirical Bayes methods. An estimation of the parameter is obtained by using Markov Chain Monte Carlo simulations of its posterior distribution. The various quantiles of these simulations provide different estimations of . The proposed methodology is validated in the sixth chapter by its application on synthetic data and real data. A sensitivity analysis of the estimation of the parameter was done. The using of the uncertainty of this parameter to quantify the uncertainty of the spatial positions of reflectors is presented in this thesis.